The present invention relates generally to casing used for packaging food products such as sausage and the like. More particularly it is concerned with a new and improved binder system used as the bonding agent for fibrous base webs used in making reinforced casing.
Heretofore it has been the practice to make reinforced films, tubing, casings or skins for food products and the like by the encasement of bonded fibrous base papers or substrates in a film forming material. In order to withstand the treatment conditions at the time of encasement, the substrate must be bonded to possess substantial dry strength, wet strength and caustic strength as well as good absorbency. Heretofore substrates of this type have been prepared by bonding a preformed and dried paper or fibrous web with a dilute (1%) viscose solution followed by the steps of drying, regenerating the cellulose, washing and redrying. This bonding operation using the dilute viscose solution was sufficient to impart enough caustic resistance to the bonded substrate to retain its structural integrity during the casing-forming operations where treatment with a more concentrated viscose solution was carried out under highly alkaline conditions. After undergoing bonding the substrate must retain its porous, absorbent characteristics in order to permit adequate impregnation and encasement by the concentrated viscose solution. Typically the casing-forming operation includes the steps of forming the substrate into a cylindrical tube, impregnating and encasing the substrate tube with a highly caustic viscose solution, regenerating the impregnate with acid, washing to remove excess viscose and acid, and drying of the final reinforced film or casing. This process is set forth in greater detail in Underwood U.S. Pat. No. 3,135,613 entitled "Impregnated Paper Webs and Method of Making Sausage Casing Thereof", thus clarifying the sequential evolution of the base web through the bonded substrate phase and then into the reinforced casing.
The tubular casings produced in the manner set forth possess enough strength, dimensional stability and burst resistance to be particularly well suited for enclosing meat and other food products that are injected into the interior of the tubes under pressure. They thereby provide firm uniform enclosures for well known products such as sausage, bologna and the like as well as other food products.
Various patents subsequent to the aforementioned U.S. Pat. No. 3,135,613 have discussed the use of alternative materials for bonding the paper webs to provide appropriate bonded substrates. In selecting bonding materials other than the commercially employed acid-regenerated dilute viscose, it is important that the bonding materials meet both the processing and performance requirements of the food casings to be produced therefrom. Additionally, the fibrous base web to which the bonding agent is to be applied must exhibit sufficient strength to withstand the stresses exerted during both the bonding and coating operations. The amount of bonding agent employed should not interfere with subsequent viscose penetration during the casing manufacturing process such that there is a loss of strength in the casing, or that there is a detrimental effect on the appearance of the casing. Also, the bonding agent should be one which will not cause the substrate to become discolored during exposure to the conditions of the casing forming process. In U.S. Pat. No. 3,484,256 to Chiu et al it is suggested that the dilute viscose bonding treatment be replaced by a bonding agent that consists of a mixture of a cationic thermosetting resin and a polyacrylamide resin. A bonding mixture of a cationic alkaline curing resin and carboxymethyl cellulose has been disclosed by Conway in U.S. Pat. No. 3,468,696 as a substitute wet strength bonding treatment. U.S. Pat. Nos. 3,640,734, 3,640,735 and 3,679,437 to Oppenheimer et al teach the formation of substrates using insolubilized poly(vinyl alcohol) as a wet strength sizing agent while the Jones et al U.S. Pat. No. 4,218,286 teaches the use of a three component binder mixture to obtain improved alkaline wet strength and good absorbency.
The aforementioned binder materials, whether used alone or in combination, frequently provide some of the desired characteristics but not all of those characteristics. For example, the use of poly(vinyl alcohol) having a degree of hydrolysis at about 85% will provide low to moderate levels of dry tensile strength but poor wet tensile, caustic strength and absorption characteristics. Conversely, the use of various film forming materials such as hydroxyethyl cellulose in conjunction with appropriate cross linking agents, such as dialdehyde cross linkers, will have the opposite effect from that achieved by the poly(vinyl alcohol). They exhibit good wet tensile strength and absorption characteristics but relatively poor dry tensile and alkaline strength. Unfortunately, mixtures of these materials also fail to provide all of the desired characteristics.
It is therefore an object of the present invention to provide a new and improved binder system capable of imparting those characteristics heretofore associated with the dilute (1%) viscose bonded material but without the attendant environmental problems associated with the use of viscose. Included in this object is the provision for a binder system that will provide high alkaline strength combined with excellent wet strength and modulus, good absorbency and extensibility, a lack of discoloration when combined with a subsequent viscose treatment and high penetration of the casing forming material into the bonded web. Additionally, the binder material advantageously provides these features in the context of selected binder characteristics that must be employed to achieve the requisite properties on both the bonded paper and the casing.
Other advantages will be in part obvious and in part pointed out more in detail hereinafter.
These and related objects and advantages are obtained by providing a bonded, porous, fibrous sheet material for use in the manufacture of food casings and the like comprising a fibrous base web containing up to about 10% by weight of a specific bonding agent. That agent comprises a solution containing a particular type of thermoplastic poly(vinyl alcohol) and a resin. The system is effective to impart high alkaline or caustic strength to the base web material provided the poly(vinyl alcohol) is both fully hydrolyzed (i.e., at least 98% hydrolyzed) and of high molecular weight (i.e., having a solution viscosity at 4% solids of about 40 centipoises or greater). The ratio of poly (vinyl alcohol) to resin should be greater than 1:1 by weight in order to achieve the appropriate combination of features that include high dry strength and alkaline strength coupled with high wet strength and absorption.
The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others and the article possessing the features, properties and relation of elements exemplified in the following detail disclosure.